1. Field of the Invention
The present invention relates to a focus detection apparatus, focus detection method, and image sensing apparatus, and more particularly, to a focus detection apparatus and focus detection method used in an image sensing apparatus, such as a digital still camera, capable of detecting a focus state of a photographing lens based on an image obtained from an image sensor for image sensing.
2. Description of the Related Art
Regarding systems for detecting a focus state of a photographing lens in a digital camera which photographs using an image sensor, an apparatus which performs pupil division-based focus detection using a two-dimensional sensor is disclosed in Japanese Patent Laid-Open No. 58-24105 (p. 2, FIG. 1), where the two-dimensional sensor has a microlens formed in each pixel. In the apparatus disclosed in Japanese Patent Laid-Open No. 58-24105, a photoelectric converter in each pixel of the image sensor is divided into multiple parts and the divided photoelectric converter is configured to receive a luminous flux passing different areas of a pupil of the photographing lens via the microlens.
Also, in Japanese Patent Laid-Open No. 2005-106994 (p. 7, FIG. 3), the present inventor discloses a solid-state image sensing apparatus which performs pupil division-based focus detection using a CMOS image sensor (solid-state image sensing apparatus) used in a digital still camera. With the solid-state image sensing apparatus disclosed in Japanese Patent Laid-Open No. 2005-106994, in some of a large number of pixels in the solid-state image sensing apparatus, the photoelectric converter is divided into two parts to detect focus state of the photographing lens. The photoelectric converter is configured to receive a luminous flux passing a predetermined area of a pupil of the photographing lens via the microlens.
Also, Japanese Patent Laid-Open No. 5-127074 (p. 15, FIG. 34) discloses a technique for detecting focus state after deforming a specific filter contained in a camera according to an aperture ratio, exit pupil position, and amount of image displacement and adapting the deformed filter to an object image.
However, in detecting focus state by dividing the photoelectric converter of each pixel into two light-receiving areas, since vignetting is caused in the luminous flux by a lens frame of the photographing lens and the like, it is difficult for each light-receiving area of the photoelectric converter to maintain photosensitivity characteristics. Consequently, it is difficult to maintain agreement of photosensitivity characteristics, that is, agreement in blur shape between a pair of images.
Thus, there is poor agreement between an image generated by a luminous flux passing through a first region of a pupil of the photographing lens and an image generated by a luminous flux passing through a second region different from the first region. Consequently, the inventions disclosed in Japanese Patent Laid-Open Nos. 58-24105 and 2005-106994 have a problem in that accurate focus detection is not possible on peripheries of the photographic screen if correlation calculation is performed based on two images generated by luminous fluxes passing through different regions on the pupil of the photographing lens.
On the other hand, the invention disclosed in Japanese Patent Laid-Open No. 5-127074 describes a method for deforming the specific filter contained in the camera according to a defocus amount and thereby performing convolution integration. However, since it is necessary to perform convolution integration for each defocus amount, the method requires massive amounts of calculation. Thus, the method has a problem in that it is difficult to ensure low costs and space savings when implementing the convolution integration in a realizable arithmetic processing circuit.